1. Field of the Invention
This invention relates to an orthopedic instrument for guiding a saw blade for shaping the distal end of a human femur to receive an endoprosthetic femoral component.
2. Description of the Prior Art
Instruments used to prepare bone surfaces and to place femoral endoprosthetic components in the knees perform two basic functions.
First is the control of power and other tools to provide accurate fixation surfaces of bone that match implant geometry. Second is to position fixation surfaces relative to bone and soft tissue architecture to appropriately orient the prosthetic component.
In addition, instruments must provide the necessary flexibility to accommodate the variations in geometry and surgical complications that are encountered within the patient population. They must also meet the disparate needs of surgeons who wish to follow different methodologies when performing the surgical replacement of the knee.
Conventional femoral components usually have five planar fixation surfaces which match the bone to the implant. Thus the femur must be prepared to have a distal cut surface, a posterior cut surface, an anterior cut surface, an anterior chamfer cut surface and a posterior chamfer cut surface.
Some existing femoral components have marginally different forms of fixation surface, ie. no posterior chamfer, or the anterior chamfer surface formed as a curved surface rather than a flat one. In general it has been found that flat surfaces are nevertheless advantageous since they are easier to prepare using oscillating saws.
A number of different surfaces can be used to control the positioning of the essentially planar blades of front or side cutting oscillating saws for shaping the planar surfaces.
Flat metallic blocks on which the saw blade is rested, obviously rely to some extent on the skill of the surgeon to avoid tilting of the saw blade, as may happen when the saw encounters a localized harder section of bone, or when the saw blade has a long travel beyond the guide surface.
Slots having small clearance relative to the thickness of the saw blade may also be used. In general these offer better control of the saw blade than blocks, but they can impede visibility at the operative site. Simple slots do not provide clearance for the tooth set on the saw blade, but a number of solutions have been proposed to this problem. These include variable thickness slots formed by assemblies of elements.
The slot is temporarily made deeper to allow passage of the saw blade teeth and subsequently reduced to more closely hold the main body of the blade in position. Alternatively, the slot is made open ended on one side so that the blade may be introduced into it from the side without having to pass the teeth through the slot. Variations in the design of the saw blade itself have also been used. These may have zero net set on the teeth or provide a local clearance behind the teeth so that the total blade and tooth form can be passed through a close clearance slot.
Block type cutting guides are shown in U.S. Pat. Nos. 4,474,177, 4,487,203, 4,502,483, 4,524,766 and 4,567,885.
Fulcrum type cutting guides are described in U.S. Pat. No. 4,718,413 and also in U.S. Pat. No. 4,892,093. These consist of an upper and a lower guide surface which are linearly separated along the plane of intended cut by the saw blade. By providing a separation between the two surfaces the saw blade, including its tooth set, may be introduced between the two surfaces and then biased against them to control the cutting plane. The separation of the guide surfaces normal to the plane of operation of the saw blade is matched to the thickness of the saw blade. The choice of orientation of the guide surfaces is chosen so that any deviation by the surgeon in maintenance of the contact between the saw blades and either of the guide surfaces results in conservative removal of bone, which may be corrected subsequently. The guide of U.S. Pat. No. 4,892,093 sits on the already prepared distal femur and provides for the cutting of four additional cuts.
The femoral components may be located with six degrees of freedom relative to the patients femoral geometry. These can be expressed in a cartesian manner relative to orthogonal anatomical reference planes.
Angulation: PA1 Linear Position:
Varus-Valgus, PA2 Flexion-Extension, PA2 Internal-External Rotation. PA2 Inferior-Superior, PA2 Anterior-Posterior, PA2 Medial-Lateral.
To position the component on the bone, a number of datum features of the patients anatomy and their relative location as controlled by soft tissue structures at the knee may be utilized.
Two major schools of though exist as to the optimum method to provide consistent functional placement. The first is independent femoral anatomical placement. In this method the femoral component is positioned on the femur by referencing datum features on the femur itself.
The second is referenced to the tibial position. In this method the position of the femoral component is controlled relative to the proximal cut of the tibia. The ligaments and other soft tissue structures at the knee joint will in this case affect the femoral components position. The positional referencing, according to different methodologies, is performed surgically prior to placing the femoral component.
A third method is varus-valgus and flexion-extension. Angulation of the component in planes is usually performed simultaneously. The reference datum is either the femoral shaft or the line joining the center of the knee and the hip joints. Two major methods for accomplishing this are currently used.
First is intramedullary alignment. A rod is introduced through the center of the knee into the intramedullary space and passed up the inside of the femur to the internal isthmus, picking the axis of the femoral shaft. This technique has been found to be very reliable, but is thought by some surgeons to be overly invasive and in patients where there is excessive bowing of the femoral shaft, or where the intramedullary space is blocked, for example by a long stemmed hip implant, it may not be available.
The second is extramedullary alignment. An external guide rod is aligned with the anterior cortex of the femur, or from the center of the knee to the femoral head.
The posterior condyles of the femur are used in the anatomical approach. In the referenced technique the internal-external rotation is controlled by balancing of the flexion gap so that the medial and lateral compartments of the joint are equally spaced or tensed.
Inferior-Superior positioning is controlled in the anatomical approach by a fixed amount of bone being resected from the distal femur. The amount of resection is normally the same as the thickness of the distal portion of the implant component where bone stock has not been eroded away. In the referenced technique the amount of bone to be removed is adjusted relative to the proximal tibial cut to allow for the total thickness of both the femoral and tibial components.
In the anatomical approach the anterior-posterior position of the femoral component may be referenced to a number of alternative features at the distal femur. These include the posterior condyles, where an amount of bone is resected from the posterior condyles which corresponds either to the posterior thickness of the femoral component or to some proportion or fixed amount in excess of this. Alternatively, anterior features or the distal femur may act as references, usually either the anterior cortex or the deepest point of the patella groove. In cases where a large intramedullary stem is to be used, the position of the femoral component may need to be chosen to match the position of the implant stem within the intramedullary canal in which it must fit. In the referenced approach a posterior resection of the femur is performed so that the flexion gap of the joint matches the thickness of the femoral and tibial components. In general all these approaches result in either an anterior or posterior cut being performed. Subsequently the opposite cut is performed so that the implant will fit between these resected surfaces.
The medial-lateral placement of the component is usually performed by eye to match the rim geometry of the resected bone surface performed by all the previous cuts. In cases where a large intramedullary stem is used, the position may be dictated by the fit of the stem into the intramedullary cavity.
Current techniques generally require the sequential use of alignment and cutting guides. In all current systems multiple cutting guides are needed to fully prepare the distal femur for the implant. Because these sequential operations require the assembly and disassembly of instrument configurations and the use of intermediate datums cut onto the bone, there are penalties in terms of time of operation and accuracy. The current invention is intended to address these inadequacies while incorporating the flexibility to allow for alternative operative approaches to be used in placement of the femoral component.